The invention relates to Niobium (Nb)-silicide based composite compositions. In particular, the invention relates to Nb-silicide based composite compositions with chemistries that permit the Nb-silicide based composite compositions to find applications in turbine components.
Turbines and their components (hereinafter xe2x80x9cturbine componentsxe2x80x9d), such as, but not limited to, aeronautical turbines, land-based, turbines, marine-based turbines, and the like, have typically been formed from nickel (Ni)-based materials, which are often referred to as Ni-based superalloys. Turbine components formed from these Ni-based superalloys exhibit desirable chemical and physical properties under the high temperature, high stress, and high-pressure conditions generally encountered during turbine operation. For example, turbine components, such as an airfoil, in modern jet engines can reach temperatures as high as about 1,150xc2x0 C., which is about 85% of the melting temperatures (Tm) of most Ni-based superalloys.
Because Ni-based superalloys have provided the level of performance desired in such applications, the development of such Ni-based superalloys has been widely explored. Consequently, the field has matured and few significant improvements have been realized in this area in recent years. In the meantime, efforts have been made to develop alternative turbine component materials. These alternate materials include niobium (Nb)-based refractory metal intermetallic composites (hereinafter xe2x80x9cRMICxe2x80x9ds). Most RMICs have melting temperatures of about 1700xc2x0 C. If RMICs can be used at about 80% of their melting temperatures, they will have potential use in applications in which the temperature exceeds the current service limit of Ni-based superalloys.
RMICs comprising at least niobium (Nb), silicon (Si), titanium (Ti), hafnium (Hf), chromium (Cr), and aluminum (Al) have been proposed for turbine component applications. These silicide-based RMICs exhibit a high temperature capability that exceeds that of current Ni-based superalloys. Exemplary silicide-based RMICs are set forth in U.S. Pat. No.5,932,033, to M. R. Jackson and B. P. Bewlay, entitled xe2x80x9cSilicide Composite with Nb-Based Metallic Phase and Si-Modified Laves-Type Phasexe2x80x9d and U.S. Pat. No. 5,942,055, to Jackson and Bewlay, entitled xe2x80x9cSilicide Composite with Nb-Based Metallic Phase and Si-Modified Laves-Type Phasexe2x80x9d.
Some known Nb-silicide based compositesxe2x80x94including silicide-based RMJCsxe2x80x94possess adequate oxidation resistance characteristics for turbine applications. These materials have compositions within the following approximate ranges. 20-25 atomic percent titanium (Ti), 1-5 atomic percent hafnium (Hf), and 0-2 atomic percent tantalum (Ta), where the concentration ratio (Nb+Ta):(Ti+Hf) has a value of about 1.4; 12-21 atomic percent silicon (Si), 2-6 atomic percent germanium (Ge), and 2-5 atomic percent boron (B), where the sum of the Si, B, and Ge concentrations is in the range between 22 atomic percent and 25 atomic percent; 12-14 atomic percent chromium (Cr) and 0-4 atomic percent iron (Fe), where the sum of the Fe and Cr concentrations is between 12 atomic percent and 18 atomic percent; 0-4 atomic percent aluminum (Al); 0-3 atomic percent tin (Sn); and 0-3 atomic percent tungsten (W). Other known Nb-based silicide compositesxe2x80x94including silicide-based RMIC materialsxe2x80x94have adequate creep-rupture resistance for turbine component applications. These materials have compositions within the following approximate ranges: 16-20 atomic percent Ti, 1-5 atomic percent Hf, and 0-7 atomic percent Ta, where the concentration ratio (Nb+Ta):(Ti+Hf) has a value of about 2.25; 17-19 atomic percent Si, 0-6 atomic percent Ge, and 0-5 atomic percent B, where the sum of the Si, B, and Ge concentrations is in the range between 17 atomic percent and 21 atomic percent; 6-10 atomic percent Cr and 0-4 atomic percent Fe, where the sum of the Fe and Cr concentrations is in the range between 6 atomic percent and 12 atomic percent; 0-4 atomic percent Al; 0-3 atomic percent Sn; 0-3 atomic percent W; and 0-3 atomic percent Mo. In addition, other known Nb-silicide based compositesxe2x80x94including silicide-based RMIC materialsxe2x80x94have adequate fracture toughness for turbine component applications. These materials contain greater than or equal to about 30 volume percent of metallic phases present in such components.
Although the above Nb-silicide based composite alloys and Nb-silicide based RMIC materials possess beneficial mechanical and chemical properties, they do not adequately balance oxidation resistance properties with toughness and creep resistance properties. Thus, a single Nb-silicide based RMIC alloy material composition that can provide adequate creep, oxidation resistance, and toughness for turbine component applications is currently not available.
While the oxidation performance and creep-rupture resistance for turbine component applications of known RMICs are desirable, these materials and their properties may still be further improved for turbine component applications. For example, the chemistries and compositions of the RMIC material may be modified to enhance oxidation resistance for applications that subject the turbine component to high stresses at temperatures ranging from about 1300xc2x0 F. to about 1700xc2x0 F. (about 700xc2x0 C. to about 925xc2x0 C.) over extended periods of time.
Therefore, what is needed is a Nb-silicide based RMIC material having a composition, chemistry, and properties that are suitable for various applications such as, but not limited to, turbine components, in which high stresses at elevated temperatures are encountered over long periods of time.
Accordingly, one aspect of the present invention is to provide a turbine having at least one component formed from a niobium silicide refractory intermetallic composite comprising: between about 14 atomic percent and about 26 atomic percent titanium; between about 1 atomic percent and about 4 atomic percent hafnium; up to about 6 atomic percent tantalum; between about 12 atomic percent and about 22 atomic percent silicon; up to about 5 atomic percent germanium; up to about 4 atomic percent boron; between about 7 atomic percent and about 14 atomic percent chromium; up to about 3 atomic percent iron; up to about 2 atomic percent aluminum; between about 1 and about 3 atomic percent tin; up to about 2 atomic percent tungsten; up to about 2 atomic percent molybdenum; and a balance of niobium.
A second aspect of the present invention is to provide a niobium silicide refractory intermetallic composite adapted for use in a turbine component. The niobium silicide refractory intermetallic composite comprises: between about 14 atomic percent and about 26 atomic percent titanium; between about 1 atomic percent and about 4 atomic percent hafnium; up to about 6 atomic percent tantalum; between about 12 atomic percent and about 22 atomic percent silicon; up to about 5 atomic percent germanium; up to about 4 atomic percent boron; between about 7 atomic percent and about 14 atomic percent chromium; up to about 3 atomic percent iron; up to about 2 atomic percent aluminum; between about 1 and about 3 atomic percent tin; up to about 2 atomic percent tungsten; up to about 2 atomic percent molybdenum; and a balance of niobium, wherein a ratio of a sum of atomic percentages of niobium and tantalum present in the niobium silicide refractory intermetallic composite to a sum of atomic percentages of titanium and hafnium present in the niobium silicide refractory intermetallic composite has a value between about 1.4 and about 2.2 (i.e., 1.4 less than (Nb+Ta):(Ti+Hf) less than 2.2).
A third aspect of the present invention is to provide a turbine component formed from a niobium silicide refractory intermetallic composite, comprising: between about 14 atomic percent and about 26 atomic percent titanium; between about 1 atomic percent and about 4 atomic percent hafnium; up to about 6 atomic percent tantalum; between about 12 atomic percent and about 22 atomic percent silicon; up to about 5 atomic percent germanium; up to about 4 atomic percent boron; between about 7 atomic percent and about 14 atomic percent chromium; up to about 3 atomic percent iron; up to about 2 atomic percent aluminum; between about 1 and about 3 atomic percent tin; up to about 2 atomic percent tungsten; up to about 2 atomic percent molybdenum; and a balance of niobium.
Refractory materials can undergo a type of oxidation often referred to as a xe2x80x9cpestingxe2x80x9d at temperatures in a range from about 1300xc2x0 F. (about 700xc2x0 C.) to about 1700xc2x0 F. (about 925xc2x0 C.). This type of refractory material oxidation is characterized by the inability of a slow-growing, protective oxide scale to form, due to kinetics of diffusion, which are characteristically slow for these materials in this temperature range. As a result of the lack of such a protective scale, oxygen can penetrate the refractory material structure at both interfacial regions and through the lattice structure of the material, thus embrittling the underlying substrate. The embrittled layer can fracture during thermal cycling. Such fracture leads to rapid material loss and ultimately causes the structure of the refractory material to disintegrate.
As disclosed in the present invention, the oxidation characteristics of refractory materials can be enhanced by the addition of several elements that have additional metallic and Laves-type phases. In the present invention, Laves-type phases preferably comprise up to about 20 volume percent of the Nb-silicide RMICs. Metallic phases preferably comprise at least 25 volume percent of the Nb-silicide RMICs. For example, if the titanium (Ti) content in a refractory material is maintained at a certain level, the performance and characteristics of the refractory material can be improved. If at least one of germanium (Ge) and tin (Sn) are added to the refractory material, the loss of refractory material due to pesting oxidation can be reduced.
In the present invention, a niobium (Nb)-silicide based alloy composite comprising a Nb-silicide refractory metal intermetallic composite (hereinafter xe2x80x9cRMICxe2x80x9ds), which overcomes the undesirable refractory material characteristic of pesting type oxidation, is described. The Nb-silicide RMIC described herein possesses a composition that provides the necessary balance between oxidation characteristics and mechanical properties. The Nb-silicide RMIC comprises: between about 14 atomic percent and about 26 atomic percent titanium; between about 1 atomic percent and about 4 atomic percent hafnium; up to about 6 atomic percent tantalum; between about 12 atomic percent and about 22 atomic percent silicon; up to about 5 atomic percent germanium; up to about 4 atomic percent boron; between about 7 atomic percent and about 14 atomic percent chromium; up to about 3 atomic percent iron; up to about 2 atomic percent aluminum; between about 1 and about 3 atomic percent tin; up to about 2 atomic percent tungsten; up to about 2 atomic percent molybdenum; and a balance of niobium. In one embodiment of the present invention, the ratio of a sum of atomic percentages of niobium and tantalum present in the niobium silicide refractory intermetallic composite to a sum of atomic percentages of titanium and hafnium present in the niobium silicide refractory intermetallic composite has a value between about 1.4 and about 2.2 (i.e., 1.4 less than (Nb+Ta):(Ti+Hf) less than 2.2). The atomic percent values given for each element are approximate unless otherwise specified.
Nb-silicide RMICs, as embodied by the invention, exhibit oxidation and rupture resistance characteristics provided by the addition of titanium (Ti), geranium (Ge) and Tin (Sn). The Nb-silicide RMICs disclosed in the present invention can be used to form turbine components such as, but not limited to, buckets, blades, rotors, nozzles, and the like for applications in land-based turbines, marine turbines, aeronautical turbines, power generation turbines, and the like.